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<div class="titlepage"><div><div><h3 class="title">
<a name="phoenix.modules.function"></a><a class="link" href="function.html" title="Function">Function</a>
</h3></div></div></div>
<div class="toc"><dl><dt><span class="section"><a href="function/adapting_functions.html">Adapting
        Functions</a></span></dt></dl></div>
<p>
        The <code class="computeroutput"><span class="identifier">function</span></code> class template
        provides a mechanism for implementing lazily evaluated functions. Syntactically,
        a lazy function looks like an ordinary C/C++ function. The function call
        looks familiar and feels the same as ordinary C++ functions. However, unlike
        ordinary functions, the actual function execution is deferred.
      </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">phoenix</span><span class="special">/</span><span class="identifier">function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
        Unlike ordinary function pointers or functor objects that need to be explicitly
        bound through the bind function (see <a class="link" href="bind.html" title="Bind">Bind</a>),
        the argument types of these functions are automatically lazily bound.
      </p>
<p>
        In order to create a lazy function, we need to implement a model of the
        <a href="http://www.boost.org/doc/libs/release/libs/fusion/doc/html/fusion/functional/concepts/poly.html" target="_top">Polymorphic
        Function Object</a> concept. For a function that takes <code class="computeroutput"><span class="identifier">N</span></code> arguments, a model of <a href="http://www.boost.org/doc/libs/release/libs/fusion/doc/html/fusion/functional/concepts/poly.html" target="_top">Polymorphic
        Function Object</a> must provide:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            An <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
            that takes <code class="computeroutput"><span class="identifier">N</span></code> arguments,
            and implements the function logic. This is also true for ordinary function
            pointers.
          </li>
<li class="listitem">
            A nested metafunction <code class="computeroutput"><span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">Signature</span><span class="special">&gt;</span></code> or nested typedef <code class="computeroutput"><span class="identifier">result_type</span></code>,
            following the <a href="http://www.boost.org/doc/libs/release/libs/utility/utility.htm#result_of" target="_top">Boost.Result
            Of</a> Protocol
          </li>
</ul></div>
<p>
        For example, the following type implements the FunctionEval concept, in order
        to provide a lazy factorial function:
      </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">factorial_impl</span>
<span class="special">{</span>
    <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>

    <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">Arg</span> <span class="keyword">const</span> <span class="special">&amp;)&gt;</span>
    <span class="special">{</span>
        <span class="keyword">typedef</span> <span class="identifier">Arg</span> <span class="identifier">type</span><span class="special">;</span>
    <span class="special">};</span>

    <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">&gt;</span>
    <span class="identifier">Arg</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Arg</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">n</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="special">(</span><span class="identifier">n</span> <span class="special">&lt;=</span> <span class="number">0</span><span class="special">)</span> <span class="special">?</span> <span class="number">1</span> <span class="special">:</span> <span class="identifier">n</span> <span class="special">*</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)(</span><span class="identifier">n</span><span class="special">-</span><span class="number">1</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
        (See <a href="../../../../example/factorial.cpp" target="_top">factorial.cpp</a>)
      </p>
<p>
        Having implemented the <code class="computeroutput"><span class="identifier">factorial_impl</span></code>
        type, we can declare and instantiate a lazy <code class="computeroutput"><span class="identifier">factorial</span></code>
        function this way:
      </p>
<pre class="programlisting"><span class="identifier">function</span><span class="special">&lt;</span><span class="identifier">factorial_impl</span><span class="special">&gt;</span> <span class="identifier">factorial</span><span class="special">;</span>
</pre>
<p>
        Invoking a lazy function such as <code class="computeroutput"><span class="identifier">factorial</span></code>
        does not immediately execute the function object <code class="computeroutput"><span class="identifier">factorial_impl</span></code>.
        Instead, an <a class="link" href="../actor.html" title="Actor">actor</a> object is created
        and returned to the caller. Example:
      </p>
<pre class="programlisting"><span class="identifier">factorial</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">)</span>
</pre>
<p>
        does nothing more than return an actor. A second function call will invoke
        the actual factorial function. Example:
      </p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">factorial</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">)(</span><span class="number">4</span><span class="special">);</span>
</pre>
<p>
        will print out "24".
      </p>
<p>
        Take note that in certain cases (e.g. for function objects with state), an
        instance of the model of <a href="http://www.boost.org/doc/libs/release/libs/fusion/doc/html/fusion/functional/concepts/poly.html" target="_top">Polymorphic
        Function Object</a> may be passed on to the constructor. Example:
      </p>
<pre class="programlisting"><span class="identifier">function</span><span class="special">&lt;</span><span class="identifier">factorial_impl</span><span class="special">&gt;</span> <span class="identifier">factorial</span><span class="special">(</span><span class="identifier">ftor</span><span class="special">);</span>
</pre>
<p>
        where ftor is an instance of factorial_impl (this is not necessary in this
        case as <code class="computeroutput"><span class="identifier">factorial_impl</span></code> does
        not require any state).
      </p>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
          Take care though when using function objects with state because they are
          often copied repeatedly, and state may change in one of the copies, rather
          than the original.
        </p></td></tr>
</table></div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas
      Heller, John Fletcher<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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